The present invention relates to a fluid power transmission such as a fluid coupling transmission or torque converter with a lock-up clutch.
In the fluid power transmission such as the torque converter, the power is transmitted from the pump impeller to the turbine runner. As a result, a difference in rotations between the pump impeller and the turbine runner cannot be avoided to lose the power. In order to prevent this power loss, therefore, there has been developed and widely adopted a torque converter which is equipped with a lock-up clutch for providing a mechanical coupling between an input side member (e.g., the front cover) and an output side member (e.g., the turbine runner hub).
If the lock-up clutch is engaged, the power is transmitted directly, as it is, from the input side member to the output side member so that no power loss is caused to improve the fuel economy or mileage.
However, the vibrations, if any, due to the torque fluctuations of the engine may be transmitted to deteriorate the riding comfort of the vehicle. In the transmission such as the torque converter for transmitting the torque through fluid, this fluid exerts a buffer action or a vibration attenuating action. If, however, the lock-up clutch is engaged, the input side member and the output side member are directly connected. With this direct connection, the engine vibrations or the torque fluctuations are transmitted as they are to the automatic transmission. Generally in the prior art, the lock-up clutch is not engaged unless the vehicle speed is higher than a predetermined level and unless the degree of throttle opening is less than a predetermined level, so that the riding comfort of the vehicle may not be deteriorated by the engine vibrations, the vibrations due to the torque fluctuations or the booming noise caused by those vibrations. As a result, the effect of improving the mileage by engaging the lock-up clutch cannot be achieved in the range of a low vehicle speed and a high degree of throttle opening.
In the lock-up range being generally executed in the prior art, on the other hand, the booming noise may occur at a relatively low vehicle speed or at a relatively high degree of throttle opening.
With a view to attenuating the vibrations in the lock-up state, therefore, there is known in the prior art a system, which is equipped on a power transmission passage through the lock-up clutch with a slip torque transmission mechanism such as the viscous coupling.
This slip torque transmission mechanism is one for transmitting the torque while allowing relative rotations and is represented by the viscous coupling. This representative is exemplified by a torque converter with a viscous coupling, as disclosed in U.S. Pat. No. 4,473,145. This torque converter, as generally designated at 1, is constructed, as shown in FIG. 12, to have its housing 2 composed of the casing of a pump impeller 3 and a front cover 2a. The torque converter 1 is equipped in the housing with the pump impeller 3, a turbine runner 4, a disc-shaped driven member 5 connected integrally to the turbine runner 4, and a disc-shaped drive member 6 made independently rotatable and axially slidable. These drive and driven members 6 and 5 have their opposed surfaces with a number of annular projections 6a and 5a which have tooth-shaped sections to interleave each other. In the clearance between these opposed driven and drive members 5 and 6, there is confined highly viscous oil such as silicone oil to constitute a viscous coupling 7.
Moreover, the disc-shaped drive member 6 is equipped on its circumferential edge with a friction member 9a, which is brought into and out of engagement with the front cover 2a by the drive member 6 when this member 6 is axially moved back and forth. Thus, a lock-up clutch 9 is provided. On the other hand, the aforementioned driven member 5 is splined to the output shaft 10 of the torque converter 1 so that it is integrally connected to the output shaft 10.
In the torque converter 1 shown in FIG. 12, therefore, most of the power is transmitted, when the lock-up clutch 9 is engaged, from the front cover 2a to the drive member 6 of the viscous coupling 7 and is further transmitted from the drive member through the viscous fluid to the driven member 5 until it is outputted to the output shaft 10 of the torque converter 1 rotating integrally with the driven member 5. In this case, the torque fluctuations and vibrations of the engine are also transmitted to the front cover 2a and the drive member 6 but are not transmitted to the driven member 5 due to the so-called "slippage" of the viscous coupling 7 and by the vibration attenuating action of the highly viscous fluid. As a result, the booming noise is prevented to prevent the deterioration of the riding comfort.
In the aforementioned torque converter having the lock-up clutch equipped with the viscous coupling, therefore, the vibrations or the like can be effectively cut by the slippage of the viscous coupling even if the lock-up range is expanded to either a low vehicle speed range, in which the engine torque highly fluctuates, or to a high degree range of the throttle opening. Despite of this advantage, however, the viscous coupling will slip even at a high-speed cruising of the vehicle, in which the torque fluctuations of the engine are so low that no slippage is required. Thus, the high loss of the torque transmission raises a problem that the mileage is accordingly degraded.